Methanol is generally produced in large facilities in which, in an intermediate step, fossil fuels such as coal or natural gas are initially converted into a synthesis gas composed of hydrogen and carbon oxides such as carbon monoxide in particular. A catalytic conversion of the synthesis gas into methanol subsequently takes place in an appropriate reactor. The methanol-containing product stream from the reactor is then cooled and fed to a separator to obtain crude methanol, with remaining residual gas being partially or completely circulated through the reactor.
Carbon monoxide and carbon dioxide with hydrogen may each be catalytically converted into methanol and possibly water in a manner known per se, wherein a molar ratio S, given by
      S    =                            n          ⁡                      (                          H              2                        )                          -                  n          ⁡                      (                          CO              2                        )                                                n          ⁡                      (            CO            )                          +                  n          ⁡                      (                          CO              2                        )                                ,of essentially 2.1, where n is expressed in moles, is sought for the most complete reaction possible of the carbon oxides and hydrogen. This molar ratio S is also referred to here and in the following discussion as the stoichiometry number. However, the synthesis gas frequently does not have such a molar ratio of essentially 2.1. In particular when synthesis gas is obtained by means of autothermal reformation, the stoichiometry number is between approximately 1.6 and 1.8, as the result of which the proportion of hydrogen is too low, and during the methanol synthesis a high proportion of carbon oxides remains. Since the remaining gas is generally circulated, the resulting increase in the gas volume to be recycled leads to drastic performance requirements for the compressors used, and requires a large quantity of catalyst for the methanol synthesis.
Various approaches are known from the prior art for approximating the desired value of the stoichiometry number by feeding hydrogen upstream from the methanol synthesis in terms of the process. For this purpose, WO 2006/126017 A1, from which the present disclosure proceeds, proposes to branch off a portion of the gas, as purge gas, remaining after the methanol synthesis, subsequent to the separation of crude methanol by condensation, and possibly to lead it through a pressure swing adsorption device (PSA). A portion of the synthesis gas is also fed prior to entry into the synthesis circuit of the PSA. The hydrogen thus obtained as well as the gas remaining after the methanol synthesis and not branched off are fed to the synthesis gas stream. The stoichiometry number may be set to the desired value by varying the proportion of the branched-off gas.
However, a disadvantage of this approach is that the carbon oxides in the gas fed to the PSA for the methanol synthesis are lost in a quantity that is proportional to the additional gain of hydrogen for the methanol synthesis. This reduces the yield of methanol relative to the source used for the generation of synthesis gas. Likewise, it is disadvantageous that, even under reaction conditions that are more favorable with regard to the stoichiometry, with the molar fractions of the carbon oxides to be applied here the methanol synthesis from the carbon monoxide preferentially takes place prior to the methanol synthesis from the carbon dioxide. As a result, the carbon dioxide from the synthesis gas is insufficiently used for the methanol synthesis, and is therefore circulated to a great extent without a synthesis reaction taking place, so that after circulation, large portions are discharged through the PSA. In addition, the increased volume flow in the circuit due in particular to the entrained carbon dioxide results in an increased requirement for catalyst volume.